Transformerless output vacuum tube audio amplifier

Abstract

An audio amplifier including a series connected vacuum tube output stage for driving an audio loudspeaker. The preferred amplifier uses the output signal as a floating reference for an intermediate stage to prevent output stage degeneration for producing balanced push-pull drive signals, thereby eliminating the need for an output transformer or AC coupling.

I claim: 
     
       1. An audio amplifier comprising: a gain stage including means for receiving an audio input from a source thereof and responsive to said input for producing first and second amplified audio signals 180 degrees out of phase from one another;   an intermediate stage including first and second intermediate unity gain sections having means for respectively receiving said first and second amplified audio signals and responsive thereto for producing respective first and second intermediate signals;   an output stage including first and second series connected vacuum tubes coupled in a push-pull configuration, and including means for setting the quiescent current of said first and second series connected vacuum tubes, and for receiving and responding to said first and second intermediate signals for producing an output signal on an output line, said output stage being operable for driving an audio loudspeaker coupled with said output line without a transformer and without a capacitor between said output stage and said audio loudspeaker;   said first and second series connected vacuum tubes being coupled for respectively receiving at the grids thereof said first and second intermediate signals and responsive thereto for respectively producing said output signal;   the cathode of said first series connected output tube being coupled with said output line so that said cathode follows said output signal resulting in degeneration of said first intermediate signal at said grid of said first series connected vacuum tube relative to said output line; and   means for compensating for said degeneration including means for providing said output signal as a voltage reference for said first intermediate unity gain section so that said output signal is superimposed only on said first intermediate signal thereby compensating for said degeneration.   
     
     
       2. The amplifier as set forth in claim 1, said gain stage including means for providing a gain of about 500 for said first and second amplified audio signals relative to said input. 
     
     
       3. The amplifier as set forth in claim 2, said gain means including vacuum tubes. 
     
     
       4. The amplifier as set forth in claim 1, said means for said first and second intermediate unity gain sections including vacuum tube cathode followers. 
     
     
       5. The amplifier as set forth in claim 4, the plate of said first intermediate vacuum tube cathode follower be coupled to said output line by means of low impedance device. 
     
     
       6. The amplifier as set forth in claim 5, said low impedance device further providing a constant voltage drop. 
     
     
       7. The amplifier as set forth in claim 5, said low impedance device means including a voltage regulator. 
     
     
       8. The amplifier as set forth in claim 6, said constant voltage drop of about 450 volts direct current. 
     
     
       9. The amplifier as set forth in claim 7, said voltage regulator including at least one shunt type zener diode. 
     
     
       10. The amplifier as set forth in claim 4, the cathode of said first intermediate vacuum tube cathode follower be coupled to said output line by way of resistance so that said output line provides a floating voltage reference to said cathode of said first intermediate vacuum tube cathode follower. 
     
     
       11. The amplifier as set forth in claim 4, the cathode of said second intermediate vacuum tube cathode follower be coupled to ground by way of resistance. 
     
     
       12. The amplifier as set forth in claim 1, said first and second intermediate signals each presenting about 200 volts peak to peak. 
     
     
       13. The amplifier as set forth in claim 1, said first and second series connected vacuum tubes respectively receiving said first and second intermediate signals by way of respective first and second grid resistance, each being greater than about 36K ohms. 
     
     
       14. The amplifier as set forth in claim 13, said first and second grid resistance each being about 100K ohms. 
     
     
       15. The amplifier as set forth in claim 13, said first and second series connected vacuum tubes being type 6C33CB triodes. 
     
     
       16. The amplifier as set forth in claim 1, further including feedback means for providing negative feedback from said output line to said gain stage. 
     
     
       17. The amplifier as set forth in claim 1, said output stage including means for operating in substantially class B mode. 
     
     
       18. The amplifier as set forth in claim 1, further including at least one pair of said series connected vacuum tubes. 
     
     
       19. The amplifier as set forth in claim 1, said means for setting said quiescent current of said first and second series connected vacuum tubes includes injecting negative bias voltage into said grids of said first and second series connected vacuum tubes, respectively. 
     
     
       20. The amplifier as set forth in claim 1, said output stage further includes means of setting quiescent voltage on said output line to zero volts by fixing said negative bias voltage on said grid of one said series connected vacuum tube and varying said negative bias voltage on said grid of other said series connected vacuum tube.